Harry Garg

Experimental Investigation of Material Removal and Surface Roughness during Optical Glass Polishing

It has been a challenge to finish optical glass surfaces due to their hard and brittle nature. Moreover, tight tolerances of surface figure and finish make polishing a more critical operation. This work reports the results of an experimental study performed for full aperture polishing of BK7 optical glass. Flat samples of borosilicate (BK7) glass are polished using an optical pitch polisher and cerium oxide (CeO2) slurry. Taguchis L9 orthogonal array is used for the design of experiments. Abrasive concentration, pressure and overarm speed are considered as variable process parameters. Polishing is performed for duration of 120 minutes for each combination of parameters. Material removal is measured using the precision weighing balance. Surface roughness was measured using a Form Talysurf PGI 120 profiler. Abrasive slurry concentration is observed to be one of the most significant parameters in the optical polishing process. It affects both the material removal rate (MRR) and the surface roughness. Pressure applied at the workpiece–polisher interface affects the MRR, but the variation of pressure is not found to affect the surface roughness significantly. Relative motion at the workpiece–polisher interface is also observed to be significant in defining the final polishing outputs.

Numerical Study on the Performance of Double Layer Microchannel with Liquid Gallium and Water

The performance of liquid gallium and water in the double layer microchannel has been analysed using three-dimensional conjugate heat transfer analysis. The effect of flow rate on the counter and parallel arrangement of each fluid is studied for three different lengths. Furthermore, cooling capability of liquid gallium and water is compared at the same length with flow rate and pumping power as governing parameters. The performance of fluid was judged on the basis of maximum temperature attained and minimal temperature variations at the heated region. Interesting results have been found showing the effect of specific heat on the type of arrangement for liquid gallium with similar observation for water for low Reynolds number and relatively longer length. Among liquid gallium and water, above certain pumping power use of liquid gallium is found to be favourable for a shorter length of the double layer microchannel. Furthermore, the range of flow rate and pumping power showing superior performance with water was found to increase with the length.

Analysis and optimization of parameters in optical polishing of large diameter BK7 flat components

ABSTRACT Optical polishing and nanofinishing of glass substrates has gained impetus owing to the continuously increasing applications of precise optical components, but still, the process is elusive and skill dependent as several unaccounted parameters induce uncertainties. This study is an effort toward deterministic surface quality of glass substrates in polishing, based upon analysis and optimization of some pronounced parameters. In the experiments, BK7 glass samples (100 mm dia.) were polished for 3 hours using Ceria slurry on a conventional eccentric one arm spindle polisher employing a polyurethane pad. The experiment was designed as per Taguchi’s L9 orthogonal array for a comprehensive analysis. Three control parameters – relative velocity, normal force (pressure), and abrasive size – were varied and the resultant average roughness value (Ra), measured by a surface profilometer (Talysurf PGI-120), constituted the response. Kinematic analysis, using existing literature, yielded velocity and MALVERN ZETASIZER measured average abrasive size using dynamic light scattering principle. Relative velocity was observed to be the most significant parameter and normal force the least significant. Optimum combination was identified by deriving a deterministic relation for the best surface finish in the given time. Ra values as low as 6.8 nm in the experiment runs and 4.4 nm in the validation experiment were obtained.

Material removal characteristics of full aperture optical polishing process

ABSTRACT Precision surfaces of optical grade have been in great demand for various applications such as high-power laser systems, astronomical reflecting telescopes glass mirrors, folding mirrors of avionics displays, reflectors, guides for transmission of hot and cold neutron beams for neutron exploration setups, electronic substrate, display covers and substrates for biomedical imaging and sensing, etc. Generation of such surfaces has been a challenge; particularly the polishing operation of optical fabrication process is quite critical which determines the final surface quality. To achieve the required optical surface parameters, a good control and systematic understanding of polishing process and its parameters are required. However, the conventional or full aperture optical polishing process still depends on operators skills to achieve the target surface quality. To exploit the process to the extent, it is must to have a scientific understanding of material removal behavior of the polishing process, which will lead to the process becoming deterministic. This article has attempted to address this issue. Authors have summarized different material removal theories and discussed various mathematical models as proposed by researchers so far. Attempt has been made to come up with knowledge gaps which are required to be bridged in future.

Full Aperture Optical Polishing Process: Overview and Challenges

The aim of advanced optical fabrication is to produce highly accurate optical surface with better reproducibility. It demands a good control and systematic understanding of the process and its parameters. Optical polishing process defines the final surface figure and finish of the component. Controlled amount of finishing forces and material removal rate are necessary for polishing of brittle materials. However, the conventional or full aperture polishing process still depends on the operator’s skills to achieve the desired surface figure and finish. The process may be well optimized at individual manufacturing setups but there appears to be a little prediction about polishing outputs. Thus, it is essential to study the fundamental mechanisms of material removal during polishing in order to achieve the accurate prediction of process outputs. This paper reviews the work carried out in the area of full aperture optical polishing.

A hybrid fabrication approach and profile error compensation for silicon aspheric optics

Aspheric optics is widely used for many optical applications due to their advantages, that is, light weight, cost-effectiveness and efficiency. There are many fabrication challenges which affect the quality of aspheric optics used for infrared-based applications. Diamond turning is one of the most suitable techniques for fabrication of infrared aspheric lens with high profile accuracies, due to its deterministic approach. However, for optics with large sag value, multiple machining cycles are required to make the best fit surface. Repeated machining cycles result in generation of inherent stresses leading to subsurface deformation and poor quality. In this study, hybrid approach of grinding and machining is proposed for fabrication of silicon infrared optics in large volume. The proposed approach results in reduced fabrication time and subsurface deformation with improved surface quality and tool life. The profile accuracy after compensation of profile error (Pt) is 0.21 µm and surface roughness (Ra) 10.5 nm is achieved.

Experimental and Numerical Analysis of Micro-Scale Heat Transfer using Carbon based Nanofluid in Microchannel for Enhanced Thermal Performance

The existing heat transfer technologies suffer from numerous limitations and are poor in high performance and high heat dissipation. Liquid cooling using microchannels and nanofluids work with the increased surface area and minimum thermal resistance. Many researchers showed that nanofluids, particularly with carbon based materials, enhance heat transfer rate. In today era, in the case of microelectronics, small miniaturized heat sinks with high heat transfer are being developed, called micro-channel heat sinks (MCHS). The proposed work is concerned about the heat transfer behavior of aqueous suspensions of CNT nanofluids flowing through the triangular shaped microchannel. Significant enhancement of the convective heat transfer is observed and the enhancement depends on the flow conditions i.e. nusselt number, microchannel channel length, nanoparticles concentration. Particle re-arrangement, shear induced thermal conduction enhancement, reduction of thermal boundary layer due to the presence of nanoparticles, as well as the very high aspect ratio of CNT nanofluids are proposed to be possible mechanisms. Results show that thermal boundary layers distorted due to use of carbon based nanofluids and heat transfer coefficient increases about three times as compared to water.

A Novel Experimental Approach for Thermal Stress Measurement in Elastomerically Mounted Optics of Head-Up Display System

Head-up display (HUD) is an opto-electro-mechanical system being used in the cockpit of aircrafts as a see-through display. Mounting and stability of optical components are critical to the opto-mechanical design of high performance display systems. The self-weight deflections and widely varying coefficient of thermal expansion (CTE) between components of field flattener lens mount (FFL) can lead to the development of stress that is subjected to large changes in temperature resulting in lens cracking or catastrophic failure without application of any mechanical load. Therefore, in terms of performance, reliability and cost of the system, it is very important to know the stress developed in the lens. This paper aims at providing a state-of-the-art experimental thermal stress analysis of elastomerically mounted optics. The major research methodologies developed in each of these aspects are presented with detail. From the investigation of stress results, experimental stress values correspond to 80%-90% range with analytical solution and numerical analysis results. So an alternate hypothesis was accepted from the validation of the results. Interestingly, the thermal stress developed in the FFL lens remained intact without exceeding the applicable tensile stress tolerances based on Cromptons suggestion. Hence, the reliability and quality are assured for optical mounts in head-up display and similar systems for aircraft application.

Performance study of magnetic cooling system using kerosene based ferrofluid under magnetic field effect

Thermal management of modern electronic circuitry is important as it governs the reliability and life of any miniaturized electronic circuit. The active cooling techniques are not efficient in removing the high heat load generated by these miniature devices. The passive cooling is the next generation cooling. This paper is an attempt to measure the performance using magnetic cooling passive system in terms of heat transfer coefficient using kerosene based ferrofluid and to evaluate the effect of parameters like temperature and pressure head of kerosene based ferrofluid. COMSOL Multiphysics modeling software has been used for analysis. The results after numerical analysis showed that the convective heat transfer coefficient increased with the use of kerosene based ferrofluid as compared to natural convection coefficient.

Numerical analysis of different shapes of microchannel for miniature cooling system

Conjugate heat transfer in rectangular, trapezoidal and rectangular-trapezoidal combined microchannels is investigated for hydro-dynamically fully developed, single-phase, laminar flow with no-slip conditions. Hydraulic diameter of all geometries is so adjusted so that for given aspect ratio of the rectangular microchannel the other two geometries have same hydraulic diameter. Various physical quantities heat transfer coefficient, Temperature gradient has been examined to deduce the effective cooling in the microchannel. Simulation has also been done for various velocities to have a look on effectiveness of the microchannel.